NPB-9507 Weekly Cruise Report #2 ...
NBP-9507 Cruise Report
Week Two: November 3-9, 1995
During the second week of NBP9507, we worked in Bransfield Strait where
we acquired nearly 1,000 nautical miles of swath bathymetry, lowered
the ZAPS sled 13 times, acquired three water sample rosettes, retrieved
one sediment core, and rendezvoused with the Polar Duke.
At the end of last week we were heading east into the eastern
Bransfield Basin, east of Bridgeman Island. We turned the ship to a
course of 050 degrees and ran straight down the axis of a rather knobby
ridge that stretched about 30 km in a straight line and attains a
height of about 1000 m from a surrounding seafloor of 2 to 2.5 km. We
surveyed the axis of the eastern Bransfield Basin towards Clarence
Island where we turned and ran a parallel course to the southwest. We
ran over a potential ZAPS sled station and surveyed a bit more. We then
took a station and proceeded to a deep spot at the end of the 30 km
ridge. We next steamed out of Bransfield Basin passing between Elephant
and Clarence Islands where we launched the second Whitworth buoy at 60
degrees 50'S, 54 degrees 04'W. We found a nice flat spot at just about
1000 m northwest of the eastern end of Elephant Island. We then turned
onto a course of 330 degrees heading into the Scotia Sea while we
deployed the seismic streamer and balanced it by attaching 0.8 pound
lead sheets every other hydrophone. There are 5 hydrophones per channel
and 48 channels. The streamer, which looks like a bright yellow garden
hose with phone lumps every 5 m, has a 1.2 km active section. The
streamer towed just below the surface with the weight that we put on
it. There are six active bird locations although the one second one
from the ship does not respond to commands. Unfortunately, by the time
we got the streamer balanced and were about to bring it back in to put
birds on, the weather had blown up to 35 kts and we were forced to
postpone the streamer test. We then tried to Seabeam across the
possible new plate boundary and got blown off the water when the winds
went up to 45 kts. Most everyone on the ship was adamant about
returning to the relative calm of Bransfield Strait.
By 1500z on the 5th of November, we had returned to the eastern
Bransfield Basin and proceeded to take ZAPS sled stations 9 and 10. We
then returned to the central Bransfield Basin steaming past Bridgeman
Island once again. We were headed for a high heat flow location found
on a 1989 R/V Polar Duke cruise. When we approached the site, we found
the area covered in heavy pack ice. As time was now running out for our
rendezvous with R/V Polar Duke on the 9th, we felt we needed to test
the seismic streamer before Rick Pearce of ITI left the ship.
Consequently we headed back north of Bridgeman Island in hopes of
getting to the relatively ice-free waters of the eastern Bransfield
Basin. We then began to deploy the streamer with birds and then the
airguns. With our normal luck the weather began to deteriorate
abruptly. We started with six SSI GI guns in the water towed as two
bundles of three each. Immediately after getting underway with the guns
firing it was obvious that the ship's crab angle would cause the port
gun bundle to rub against the new streamer. The port guns were pulled
and since one of the starboard guns was leaking and had been shut down,
it made little sense to continue seismic work since we were not
collecting quality data. It is safe to say that the streamer appears to
be quiet and to work well. Alas, we have not been able to collect any
real data with it because of weather. So, we once again passed
Bridgeman Island and headed southwest for station work near the
previously identified zone of high heatflow where we made ZAPS and
rosette lowerings and acquired a core.
Sediment Coring
The coring device used is a modified Kastenlot corer, a three-meter
long stainless steel barrel with a square cross-section. Operatedby
Amelia Shevenell, a Hamilton College undergraduate representative of
sedimentologist Eugene Domack, and University of Texas foraminiferal
micropaleontologist Ben Sloan, the device is designed to preserve the
sediment-water interface. The initial lowering of the device failed to
retrieve any sediment, most likely because the half-ton instrument was
too light for the heavy aft winch line on which it was lowered. A
second lowering on the starboard winch penetrated the sea bottom, but a
flapper stuck in the open position and allowed the sample to slide out
as the tool was lifted from the water. Following modification of the
catcher by ASA marine tech Robert Kane, a third penetration returned
with a full three meters of mud from 1,960 meters water. The mud was
described by Shevenell and Sloan as very soft, weakly stratified
olive-gray hemipelagic mud with occasional interbedded black layers
inferred to be rich in heavy minerals, possibly manganese and/or
sulfides. The two photographed the core and took samples at ten
centimeter intervals for sedimentological and foraminiferal analyses.
Sloan extracted small subsamples of the top ten centimeters at
one-centimeter intervals to be used to evaluate the microhabitats of
foraminifera living in the mud near the sea bottom.
Rendezvous
We continued our Seabeam mapping of the region and met R/V Polar Duke
at about 0100 local on the eighth. I had allotted 3 hours for the
rendezvous, which began on calm, moonlit waters. The computer
technicians from our ship went over for emergency TLC for the Duke's
systems. Steve Stevenoski, our high school teacher went over as well to
find out how others work under far more trying conditions. The
scientific party of the Duke as well as Al Hickey, their MPC, visited
the Palmer. About two hours into the rendezvous, the winds kicked up to
almost 20 kts and the transfer party returned cold and wet. The air
tempature was down to -4 degrees C. After the transfer we continued
surveying and returned to the Great Wall where we undertook our first
ZAPS drift.
Hydrothermal Work
Work on Project S-060 (Hydrothermal Survey of Bransfield Strait) went
exceedingly well. We have carried out 19 lowerings of the ZAPS
instrument package and 7 rosette casts. These stations were distributed
along the axis of the rift between 54 degrees 30'W to 59 degrees 00'W.
This area includes the eastern basin and the majority of the central
basin. Water samples were collected for radon gas, helium isotopes,
manganese, and rare earth elements. Both filtered and unfiltered
samples were collected. Radon is being measured on the ship; the other
analyses will be carried out in shore-based laboratories. In addition
manganese profiles are being determined routinely with a flow-through
chemical sensor on the ZAPS sled. CTD information from the instrument
package and rosette are well matched. This combined data set provides
us with the best possible coverage.
Hydrography across this area is complex, especially in intermediate
waters where there are well developed temperature and salinity maxima
and a dissolved oxygen minimum. Several water masses are present and
the proportions of these waters vary across the study area. This
information will add considerably to what we know about circulation in
the Strait and communication of the waters here with other areas.
We have detected signals indicative of hydrothermal activity in several
areas, including both basins. At one of these locations in the central
basin our exploration has advanced to the site survey stage. Along a
ridge of pillow basalts in the central basin known as the "great wall"
we detected a hydrothermal plume that has a turbidity signal comparable
to that observed at the TAG mound on the Mid-Atlantic Ridge. This plume
is the first clear evidence of high-temperature, black smoker vents in
the Bransfield Strait. More detailed work at this site is planned for
later in the project.
Lawver, Klinkhammer, and the Shipboard Scientific Party
Jackson School of Geophysics»
Note: The photos and journals
contained in this website were prepared by Steven Stevenoski, Science
teacher at Lincoln High School in Wisconsin Rapids, Wisconsin